Here's what they look like from the outside:
Here's the inside layout:
Items
Every Grotto can spawn the following, with the percentage given being the chance of it spawning:
Visible items
Poke Ball: 12.5%
Great Ball: 5%
Ultra Ball: 2%
Potion: 6.25%
Super Potion: 2.5%
Hyper Potion: 1%
Repel: 6.25%
Super Repel: 2.5%
Max Repel: 1%
Hidden items
Damp Mulch: 6.25%
Growth Mulch: 6.25%
Stable Mulch: 6.25%
Gooey Mulch: 6.25%
TinyMushroom: 7.5%
Big Mushroom: 2.5%
Red Shard: 1%
Green Shard: 1%
Yellow Shard: 1%
Blue Shard: 1%
Rare Candy: 0.25%
PP Up: 0.5%
PP Max: 0.25%
Items specific to certain Grottoes
Fire/Water/Thunder/Leaf Stone: 0.25%
Route 2
Route 3 (dark grass)
Route 5
Route 6 (near Breeder)
Route 7
Route 9
Route 13 (southwest from the two Cuttable trees)
Route 18
Route 22
Route 23
Floccesy Ranch
Giant Chasm
Abundant Shrine (near Youngster)
Lostlorn Forest
Pinwheel Forest (both)
Moon/Sun/Shiny/Dusk Stone: 0.25%
Route 6 (Mistralton Cave)
Route 13 (Above the stairs near the entrance to Giant Chasm)
Abundant Shrine (right of the shrine)
Dawn Stone: 1%
Route 3 (near the pond)
Pokemon
Route 2
Nidoran♀: 2%
Nidoran♂: 2%
Granbull: 1%
Watchog: 15%
Route 3 - Dark Grass
Manectric: 4%
Pachirisu: 15%
Zebstrika: 1%
Route 3 - Pond
Venonat: 4%
Lombre: 1%
Bibarel: 15%
Route 5
Liepard: 4%
Minccino: 1%
Foongus: 15%
Route 6 - Near Breeder
Dunsparce: 1%
Woobat: 4%
Foongus: 15%
Route 6 - Mistralton Cave
Nosepass: 1%
Woobat: 4%
Foongus: 15%
Route 7
Zangoose: 7.5%
Seviper: 7.5%
Watchog: 4%
Cubchoo: 1%
Route 9
Muk: 2%
Liepard: 15%
Garbodor: 2%
Bouffalant: 1%
Route 13 - Stairs
Spheal: 1%
Drifloon: 4%
Foongus: 15%
Route 13 - Giant Chasm
Tangela: 4%
Skorupi: 1%
Foongus: 15%
Route 18
Kingler: 7.5%
Dragonite: 1%
Corsola: 7.5%
Chatot: 4%
Route 22
Pelipper: 4%
Amoonguss: 15%
Mienfoo: 1%
Route 23
Golduck: 2%
Gligar: 1%
Zangoose: 7.5%
Seviper: 7.5%
Absol: 2%
Abundant Shrine - Near Youngster
Vulpix: 1%
Bronzor: 4%
Amoonguss: 15%
Abundant Shrine - Shrine
Golduck: 4%
Swablu: 1%
Amoonguss: 15%
Giant Chasm
Clefairy: 15%
Ditto: 1%
Sneasel: 2%
Metang: 2%
Lostlorn Forest
Pinsir: 0.75%, Black 2 only
Pinsir: 0.25%, White 2 only
Heracross: 0.25%, Black 2 only
Heracross: 0.75%, White 2 only
Combee: 15%
Leavanny: 4%
Pinwheel Forest - Outer Area
Poliwhirl: 1%
Hariyama: 15%, Black 2 only
Hariyama: 1%, White 2 only
Medicham: 1%, Black 2 only
Medicham: 15%, White 2 only
Bagon: 3%
Pinwheel Forest - Inner Area
Butterfree: 4%, White 2 only
Beedrill: 4%, Black 2 only
Murkrow: 1%
Breloom: 3.75%
Amoonguss: 11.25%
Floccesy Ranch
Marill: 15%
Dunsparce: 1%
Herdier: 4%
Funfest Missions
Glameow, White 2 only
Stunky, Black 2 only
Eevee
Vaporeon
Jolteon
Flareon
Espeon
Umbreon
Leafeon
Glaceon
Note: Funfest Mission Pokemon have no percentage, as they can only be found during specific missions.
Source
Hope I helped!